The present invention relates to a positioning structure between different materials such as resin and metal that are different in degree of modification due to environmental variation. The present invention further relates to a motor assembly structure that uses the positioning structure.
A mold motor, whose stator is molded of resin, has been previously used in order to improve chemical resistance and insulation. A mold motor is disclosed in Japanese unexamined patent publication JP2002-218720A, for example.
In one type of mold motors, a metal front cover is fixed to a resin-molded stator assembly, and a rotor is rotatably supported by bearings mounted to the front cover.
FIG. 10A, FIG. 10B, FIG. 11 and FIG. 12 show a conventional example of an assembly structure of such a mold motor. FIG. 10A is a plan view of a metal front cover, FIG. 10B is a plan view of a resin-molded stator assembly, FIG. 11 is a sectional view of the front cover and the stator assembly before attaching the front cover to the stator assembly and FIG. 12 is a sectional view of the stator assembly to which the front cover is attached. FIGS. 11 and 12 omit showing parts included in the stator assembly, such as coils and claw poles, and show the whole as a resin member.
As shown in FIGS. 10A and 11, a center hole 1a is formed at the center of the front cover 1. A bearing (not shown) that supports a rotation shaft will be provided inside the center hole 1a. Four projections 1b are formed along a circle that is concentric with the center hole 1a at equal angular intervals. On the other hand, a stator assembly 2 is constructed by cascading a plurality of unit stators (not shown) that are molded of resin. Each of the unit stators consists of a coil wound around a bobbin and a pair of claw poles that sandwich the bobbin. As shown in FIGS. 10B and 11, a rotor housing 2a is formed in the center of the stator assembly 2 to hold a rotor, and an upper surface to which the front cover 1 is attached is formed so that the center area is dented by one step with respect to the circumference to form a step 2b along the boundary.
The inside diameter of the step 2b is determined to be nearly equal to a diameter of the circumscribed circle of the projections 1b. As shown in FIGS. 11 and 12, when the projections 1b are fitted inside the step 2b, the front cover 1 can be positioned to the stator assembly 2 so that the center of the center hole 1a of the front cover 1 is coincident with the center of the rotor housing 2a of the stator assembly 2.
However, the conventional positioning structure of a motor has a problem that environmental variation, such as temperature variation and humidity variation, changes the relative positional relationship between the front cover 1 and the stator assembly 2. When the change of the relative positional relationship becomes large, the rotor supported by the front cover 1 is decentered and contacts the stator assembly, which disables the rotation of the motor.
This problem will be described using FIGS. 13A through 13C, 14A and 14B. FIGS. 13A through 13C are plan views showing positional relationships of the front cover 1, which has the center hole 1a and the projections 1b, with respect to the stator assembly 2, which has the rotor housing 2a and the step 2b, FIGS. 14A and 14B are sectional views thereof. That is, according to the conventional positioning structure of a motor, the four projections are inscribed in the step 2b in normal temperature as shown in FIGS. 13A and 14A, which positions the front cover 1 to the stator assembly 2 so that the center hole 1a and the rotor housing 2a are concentric. Since expansion of the resin part of stator assembly 2 is larger than that of the metal front cover 1, the increase in temperature causes a gap between the projections 1b and the step 2b as shown in FIGS. 13B and 14B, which disenables the positioning function. Therefore, if an external force in a radial direction is added under this condition, the front cover 1 will be displaced with respect to the stator assembly 2 as shown in FIG. 13C, which decenters the rotor supported by the front cover through the rotation shaft with respect to the rotor housing 2a of the stator assembly 2. As a result, the rotor may contact the stator assembly 2 and rotation may become impossible.
Further, since the resin part deforms easily than the metal part due to not only temperature variation and humidity variation but also an effect of external force, the same fault as the above may be caused by such environmental variation. Still further, the problem of aggravation of the positioning accuracy due to environmental variation is caused in not only the motor but also a composition that combines parts of different materials.